1. Field of the Invention
This invention relates to a retention system for a mirror and, more particularly, to a retention system for a stage mirror on the wafer stage of a projection exposure device for manufacturing semiconductor wafers using a photolithography process.
2. Description of the Related Art
Conventional devices for manufacturing integrated circuits utilize a photolithographic process to reproduce a circuit pattern from a reticle, or photomask, onto the surface of a wafer. In this process, radiant energy is transmitted through non-opaque portions of the reticle, through a projection exposure apparatus, and onto a surface of the wafer coated with photosensitized silicon or other semiconductor material. Conventional devices use visible and ultraviolet light as a radiant energy source. The portions of the wafer surface that are exposed to the light are cured. The uncured portion is then removed by an acid bath.
The resulting silicon layer is used to produce one layer of a multi-layered integrated circuit. A complete integrated circuit is formed by overlaying additional silicon layers on the existing layers. The overlay process involves recoating the wafer and repeating the exposure process using reticles with different circuit patterns.
The wafers used in this process contain multiple dies arranged in rows on the wafer surface. Each die is used to produce a separate integrated circuit. During manufacturing, a first circuit layer is formed on each die by reproducing the circuit pattern from the reticle on each die surface. Further circuit layers are formed by overlaying additional circuit patterns on the first circuit layer of each die.
In one conventional device, the wafer is mounted on the surface of a computer-controlled platform, or stage, which precisely positions the dies with respect to the projection exposure apparatus. After a die is exposed, the wafer stage sequentially moves the remaining dies into position for exposure. This device is known as a step-and-repeat exposure system, or a stepper.
Stepper devices use laser interferometers, which detect the position of a target, such as a stage mirror, to precisely determine and control the position of the wafer stage during manufacturing. It has been found that the stage mirror moves relative to the wafer stage during motion transients of the stage, such as stopping, starting, and directional changes. The motion of the stage mirror results in measurement errors, known as backstep errors, as the wafer stage moves dies into position for exposure, and alignment errors, known as overlay errors, as subsequent circuit layers are applied to the dies.
In order to reduce backstep and alignment errors, stainless steel holders have been used to secure the stage mirror on the wafer stage. These conventional holders each provide point contact with the mirror, with a first holder contacting the mirror""s reflective surface and a second holder contacting the mirror""s non-reflective, or rear surface. Point contact is provided by rounded contact elements. The contact element of the second holder is spring biased to hold it in contact with the mirror surface.
There are a number of problems associated with the conventional holders. First, the holders are not stiff enough to adequately restrain the mirror. As the wafer stage moves in a first direction, the holders flex, allowing the stage mirror to move in the opposite direction, leading to positional measurement errors. Stepper systems with the stainless steel holders have been unable to provide the positional tolerances required for the manufacturing of modern integrated circuits.
Further, the conventional holders have introduced several non-linear measurement errors for which the control computer cannot compensate. The contact point of the first holder has been found to scratch the mirror""s reflective surface, causing surface imperfections. The contact point then moves over the mirror""s surface in an erratic stick/slip fashion, introducing measurement inaccuracies.
Finally, the force from the contact point of the second holder has been found to be inadequate to prevent slippage of the mirror relative to the wafer stage. The use of stronger springs to increase the biasing force of this contact point merely increases the deflection of the first holder, while not reducing the slippage of the mirror.
The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and consistent with the principles of the invention, as embodied and broadly described herein, a first aspect of the invention is a retention system for securing a mirror to a mounting surface of a wafer stage movable along an axis, wherein the mirror includes a substantially planar reflective surface, a rear surface, and an exposed surface, and wherein the reflective surface is substantially perpendicular to the axis. The retention system includes at least one first bracket adapted to engage the wafer stage and the mirror to restrain motion of the mirror relative to the wafer stage in at least a direction parallel to the axis, the at least one first bracket comprising a substantially planar contact surface adapted to engage a first one of the reflective surface and the rear surface of the mirror, and at least one second bracket adapted to engage the wafer stage and the mirror to restrain motion of the mirror relative to the wafer stage in at least a direction parallel to the axis, the at least one second bracket comprising a substantially planar contact surface adapted to engage a second one of the reflective surface and the rear surface of the mirror.
In another aspect, the invention provides a retention system including a platform movable along an axis, the platform including a mounting surface, a detection member disposed on the mounting surface, the detection member including an upper surface, a lower surface, a forward surface, and a rear surface, wherein the forward surface is substantially planar and substantially perpendicular to the axis, at least one first bracket engaging the platform and the detection member to restrain motion of the detection member relative to the platform in at least a direction parallel to the axis, the at least one first bracket comprising a first contact surface engaging one of the forward surface and the rear surface of the detection member, and at least one second bracket engaging the platform and the detection member to restrain motion of the detection member relative to the platform in at least a direction parallel to the axis, the at least one second bracket comprising a second contact surface engaging the other of the forward surface and the rear surface of the detection member.
In a further aspect, the invention provides a method of securing a mirror to a mounting surface of a wafer stage movable along an axis, the method including providing a mirror including a substantially planar reflective surface, a rear surface, and an exposed surface, wherein the reflective surface is substantially perpendicular to the axis, engaging at least one first bracket with the wafer stage and the reflective surface to restrain motion of the mirror relative to the wafer stage in at least a direction substantially parallel to the axis, and engaging at least one second bracket with the wafer stage and the rear surface to restrain motion of the mirror relative to the wafer stage in at least a direction substantially parallel to the axis.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. Additional advantages will be set forth in the description which follows, and in part will be understood from the description, or may be learned by practice of the invention. The advantages and purposes may be obtained by means of the combinations set forth in the attached claims.